In conventional processes, an unsaturated aldehyde, namely, acrolein, is hydrated with a solution in the presence of a catalyst to obtain hydroxyalkanal, namely, 3-hydroxypropanal(3-hydroxypropionaldehyde), which will be explained in the following paragraphs.
U.S. Pat. No. 2,434,110 discloses a process, in which a mineral acid, such as a sulfuric acid, is used as a homogeneous acid catalyst for the above reaction step. However, 3-hydroxypropanal retains low selectivity in this process, and thus is not produced efficiently. In addition, neither 3-hydroxypropanal is readily separated from the homogeneous catalyst, nor can the catalyst can be re-used easily.
To eliminate such a drawback, processes for improving the selectivity of 3-hydroxypropanal are proposed in the undermentioned publications.
U.S. Pat. No. 3,536,763 discloses a process, in which an acid ion exchange resin is used as an heterogeneous acid catalyst for the above reaction step.
U.S. Pat. No. 5,015,789 and U.S. Pat. No. 5,171,898 disclose processes, in which an ion exchange resin containing a phosphonate group, an amino group, or an aminophosphate group is used as a heterogeneous acid catalyst for the above reaction step.
U.S. Pat. No. 5,093,537 discloses a process, in which alumina bonding zeolite is used as a heterogeneous acid catalyst for the above reaction step.
U.S. Pat. No. 5,276,201 discloses a process, in which TiO.sub.2 carrying a phosphoric acid is used as a heterogenous acid catalyst for the above reaction step.
Also, U.S. Pat. No. 5,284,979 discloses a process, in which the above reaction step is performed using a buffer solution containing a carboxylic acid and tertiary amine in the presence of an acid catalyst.
If a resulting solution of the raw material, acrolein, has low density (i.e., lower than 20 percent by weight), 3-hydroxypropanal retains satisfactory selectivity, thereby making it possible to obtain 3-hydroxypropanal at high selectivity by the above processes.
However, the inventors of the present invention found that when an industrially advantageous high-density acrolein solution (i.e., 20 or more percent by weight) is used for the reaction in each of the above processes, the reaction product, 3-hydroxypropanal, triggers an active consecutive reaction (side reaction) because it has an aldehyde group.
In other words, the above processes have a drawback that the selectivity from acrolein to 3-hydroxypropanal, and hence the selectivity of 3-hydroxypropanal is reduced. Therefore, increasing the density of the acrolein solution does not improve the yield of 3-hydroxypropanal by the above processes, indicating that these processes are not satisfactory in terms of industrial use.